JP5942630B2 - Optical fiber manufacturing method - Google Patents

Description

  The present invention relates to a method of manufacturing an optical fiber.

  In an optical communication system, an optical fiber is used as an optical transmission path for transmitting signal light. A fiber used as a next-generation trunk-system optical transmission line is required to have low transmission loss and low nonlinearity. Annealing drawing technology is known as a technology for reducing the loss of an optical fiber. Annealing drawing technology means that when an optical fiber is produced by drawing an optical fiber preform in a drawing furnace, the optical fiber output from the drawing furnace is annealed at a temperature of 1600 ° C. or lower for a certain period of time. This is a technique that can reduce the disorder of the glass structure in the optical fiber and keep the Rayleigh scattering loss low.

  Various annealing drawing techniques have been proposed so far, for example, by optimizing the drawing speed in the drawing process, the position of the heating furnace, the length of the heating furnace, or the annealing time. It is said that transmission loss can be significantly reduced as compared with the case of no transmission. Further, it is known that the appropriate conditions for annealing drawing differ depending on the composition (viscosity) of the optical fiber preform. Therefore, appropriate conditions for annealing drawing are determined according to the composition of the optical fiber preform.

International Publication No. 00/62106 International Publication No. 2004/007383 JP 2000-335934 A JP 2004-307280 A

  In the annealing drawing technology, the OH group present in the optical fiber (especially the clad) diffuses near the core during annealing, so that the transmission loss with a wavelength of 1.38 μm derived from OH group absorption is annealed. There is a concern that it will increase compared to the case where it did not. In the trunk optical fiber, the wavelength band used is near 1.55 μm, and the wavelength band near 1.38 μm is not generally used. However, since the OH absorption has a peak at a wavelength of 1.38 μm and has a certain spread, there is a concern that an optical fiber having an extremely high OH concentration may slightly increase transmission loss due to OH absorption even at a wavelength of 1.55 μm. When this optical fiber is used in an optical transmission line including a Raman amplifier, light having a wavelength of around 1.45 μm is used as pump light for Raman amplification, so that the signal intensity at a wavelength of 1.55 μm is remarkable. It is thought to be affected by OH absorption loss. Therefore, keeping the OH absorption loss low is also useful for trunk optical fibers.

  In order to reduce the loss resulting from OH absorption, it is straightforward to reduce the OH concentration in the optical fiber preform. An optical fiber preform is generally manufactured by applying an additional cladding (jacket layer) to the surface of a core rod including a core and a part of the cladding. If the core rod in which the power of the propagating light is concentrated is sufficiently dehydrated and the OH concentration is sufficiently low, the core rod and the jacket layer are formed even if the OH concentration is higher than the core rod in the jacket portion where the relative power ratio is small. OH absorption loss can be kept low by optimizing the size ratio and reducing the ratio of the propagation light power in the jacket. Thereby, since the dehydration treatment of the jacket can be omitted, the optical fiber preform and the optical fiber can be manufactured at low cost.

  However, when annealing is performed in order to reduce the Rayleigh scattering loss, even if the size ratio between the core rod and the jacket layer is optimized and the ratio of the propagation light power in the jacket is lowered, the OH group is not removed by annealing. Since it diffuses, there is a case where the OH absorption loss cannot be sufficiently reduced.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a method capable of manufacturing an optical fiber with reduced Rayleigh scattering loss and OH absorption loss at low cost. To do.

  The optical fiber manufacturing method of the present invention is a method of manufacturing an optical fiber having a core and a cladding, and a jacket layer to be the remainder of the cladding is provided around the core rod to be a part of the core and the cladding. In a preform manufacturing process for manufacturing a fiber preform, a drawing process for manufacturing an optical fiber by drawing an optical fiber preform in a drawing furnace, a heating process for heating an optical fiber in a heating furnace, and a heating process A coating step of applying a coating layer around the optical fiber after being heated.

  Furthermore, in the optical fiber manufacturing method of the present invention, in the preform manufacturing process, the average OH concentration is less than 1 wtppb, and the OH concentration in the region where the thickness of the optical fiber after drawing from the surface is 0.03 μm is 10 wtppm or more. A jacket layer having an average OH concentration of 10 wtppm or more is provided around the core rod, and the ratio of the power of propagating light having a wavelength of 1.38 μm in the core rod equivalent region of the drawn optical fiber is 99.99975% or more. An optical fiber preform is produced. In the heating step, the temperature of the optical fiber when the optical fiber enters the heating furnace is 1400 ° C. or higher, the heating temperature of the optical fiber in the heating furnace is 2000 ° C. or lower, and the residence time of the optical fiber in the heating furnace is 0.005. ~ 1 second.

  In the optical fiber manufacturing method of the present invention, the average OH concentration around the core rod having an OH concentration of 100 to 1000 wtppm or more in the region where the thickness of the optical fiber after drawing from the surface is 0.03 μm in the preform manufacturing process. It is preferable to prepare an optical fiber preform by providing a jacket layer having a thickness of 10 to 300 wtppm or more. In the heating step, it is preferable that the heating temperature of the optical fiber in the heating furnace is 800 to 1600 ° C. or lower. In the heating step, it is preferable that the heating time of the optical fiber in the heating furnace is 0.10 to 1.00 seconds. In the drawing process, it is preferable that the average glass cooling rate from the start of the reduction of the glass outer diameter of the optical fiber preform to the time when the optical fiber exits from the drawing furnace is 500 ° C./m or more. .

  According to the present invention, an optical fiber in which both Rayleigh scattering loss and OH absorption loss are reduced can be manufactured at low cost.

It is a figure which shows the structure of the optical fiber manufacturing apparatus. It is a graph which shows the relationship between the residence time of the optical fiber in a heating furnace, and the transmission loss of the optical fiber in wavelength 1.55micrometer. It is a graph which shows the relationship between the heating temperature of the optical fiber in a heating furnace, and the transmission loss of the optical fiber in wavelength 1.55micrometer. It is a graph which shows the relationship between the residence time of the optical fiber in a heating furnace, and transmission loss increase amount (DELTA) (alpha) 1.38 derived from the OH absorption loss in wavelength 1.38micrometer. It is a graph which shows the relationship between Ra of an optical fiber preform | base_material, and transmission loss increase amount (DELTA) (alpha) 1.38 derived from OH absorption loss in wavelength 1.38 micrometers. It is a graph which shows the relationship between Ra of an optical fiber preform | base_material, and transmission loss increase amount (DELTA) (alpha) 1.38 derived from OH absorption loss in wavelength 1.38 micrometers. It is a graph which shows the relationship between the ratio of the power of the propagation light of wavelength 1.38 micrometers in the area | region equivalent to a jacket in an optical fiber, and transmission loss increase amount (DELTA) (alpha) 1.38 derived from OH absorption loss in wavelength 1.38 micrometers.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a diagram showing a configuration of an optical fiber manufacturing apparatus 1. The optical fiber manufacturing apparatus 1 manufactures an optical fiber 3 by drawing an optical fiber preform 2 made of quartz glass, and further manufactures an optical fiber 4 by applying a coating layer around the optical fiber 3. To do. The optical fiber manufacturing apparatus 1 includes a drawing furnace 11, a heating furnace 21, an outer diameter measuring device 41, a coding die 51, and a resin curing unit 31.

The drawing furnace 11 heats and softens the lower end of the optical fiber preform 2 held and supplied by the preform supply unit by the heater 12, and draws the optical fiber preform 2 to manufacture the optical fiber 3. . The inside of the core tube 13 of the drawing furnace 11 is supplied with an inert gas from an inert gas supply unit 14 via an inert gas supply path 15 to form an inert gas atmosphere. The optical fiber 3 after drawing is rapidly cooled by an inert gas to a temperature in the furnace core tube 13 of about 1700 ° C. Thereafter, the optical fiber 3 is taken out of the drawing furnace 11 from the lower part of the furnace core tube 13 and is air-cooled between the drawing furnace 11 and the heating furnace 21. For example, N ₂ gas is used as the inert gas. The thermal conductivity coefficient λ (T = 300K) of N ₂ gas is 26 mW / (m · K). The thermal conductivity coefficient λ (T = 300K) of air is 26 mW / (m · K).

The heating furnace 21 heats the optical fiber 3 output from the lower part of the drawing furnace 11 and gradually cools it at a predetermined cooling rate. The heating furnace 21 has a furnace core tube 23 through which the optical fiber 3 passes. The position of the core tube 23 is set at a predetermined interval with respect to the core tube 13 of the drawing furnace 11 so that the incoming temperature of the optical fiber 3 becomes a predetermined value. The length of the furnace core tube 23 is set so that the heating time of the optical fiber 3 in the heating furnace 21 becomes a predetermined value. The heater 22 of the heating furnace 21 heats the temperature of the center of the furnace core tube 23, that is, the temperature of the optical fiber 3 to a predetermined value. Reactor core tube 23 of the heating furnace 21, _{N 2} gas is supplied from the _{N 2} gas supply unit 24 through the _{N 2} gas supply path 25, it is _{N 2} gas atmosphere. Instead of using N ₂ gas, it is also possible to use a gas having a relatively large molecular weight such as air or Ar. When the heater 22 is a carbon heater, it is necessary to use an inert gas.

  The outer diameter measuring device 41 measures the outer diameter of the optical fiber 3 exiting from the heating furnace 21 and sends the measured outer diameter value to the control unit 44. The controller 44 controls the rotational speed of the drive motor 43 that rotationally drives the drum 42 that winds the optical fiber 4, so that the measured outer diameter of the optical fiber 3 becomes the target value. Adjust. By such feedback control, the outer diameter of the optical fiber 3 is set to a constant target value.

  The coding die 51 applies a UV resin liquid (ultraviolet curable resin) 52 to the outer periphery of the optical fiber 3 after passing through the outer diameter measuring device 41. The resin curing unit 31 cures the ultraviolet curable resin by irradiating the ultraviolet curable resin applied to the optical fiber 3 with the ultraviolet light output from the ultraviolet lamp 32. Thereby, the optical fiber 4 in which the resin coating layer is provided around the optical fiber 3 is manufactured. Note that a thermosetting resin may be applied instead of the ultraviolet curable resin, and the thermosetting resin may be heated to be cured. Then, the optical fiber 4 is wound around the drum 42 through the guide roller 61. The drum 42 is supported by a rotation drive shaft 45, and an end portion of the rotation drive shaft 45 is connected to a drive motor 43.

  The optical fiber manufacturing method of this embodiment is a method of manufacturing an optical fiber using such an optical fiber manufacturing apparatus 1. The optical fiber manufacturing method of the present embodiment includes a base material manufacturing process for manufacturing an optical fiber base material, a drawing process for manufacturing an optical fiber by drawing an optical fiber base material with a drawing furnace 11, and a heating furnace 21. The heating process which heats an optical fiber by this, and the coating process which provides a coating layer around the optical fiber after being heated in a heating process are provided.

The transmission loss of an optical fiber is generally represented by the sum of the Rayleigh scattering loss, the absorption loss of the glass itself, and the absorption loss of the additive contained in the glass. It is known that Rayleigh scattering loss is dominant in a band of 1.31 to 1.55 μm, which is a general signal light wavelength band in an optical communication system using an optical fiber as an optical transmission line. The Rayleigh scattering loss is caused by the randomness of the glass SiO ₂ network. As described in Patent Document 3 and the like, the annealing wire drawing technology cools the optical fiber by passing the optical fiber output from the wire drawing furnace through a heating furnace provided immediately after the wire drawing furnace. The speed is reduced and the Rayleigh scattering loss due to the disorder of the SiO ₂ network is reduced.

  Appropriate slow cooling conditions for the optical fiber in the heating furnace are the temperature of the optical fiber when the optical fiber enters the heating furnace (incidence temperature), the heating temperature of the optical fiber in the heating furnace, and the stay of the optical fiber in the heating furnace. Determined by time. A general single mode optical fiber was manufactured by setting each of the incoming temperature, heating time, and residence time of the optical fiber in the heating furnace, and the transmission loss at a wavelength of 1.55 μm of the optical fiber was evaluated. The results are shown in FIGS. 2 and 3.

  FIG. 2 is a graph showing the relationship between the residence time of the optical fiber in the heating furnace and the transmission loss of the optical fiber at a wavelength of 1.55 μm. Here, the incoming temperature of the optical fiber in the heating furnace was higher than 1500 ° C., and the heating temperature of the optical fiber in the heating furnace was 1400 ° C. As shown in the figure, the longer the residence time of the optical fiber in the heating furnace, the lower the Rayleigh scattering loss, and the transmission loss at a wavelength of 1.55 μm by making the residence time 0.1 seconds or longer. Can be set to 0.185 dB / km or less.

  FIG. 3 is a graph showing the relationship between the heating temperature of the optical fiber in the heating furnace and the transmission loss of the optical fiber at a wavelength of 1.55 μm. Here, the incoming temperature of the optical fiber in the heating furnace was made higher than 1500 ° C., and the residence time of the optical fiber in the heating furnace was 0.5 seconds. As shown in the figure, the higher the heating temperature of the optical fiber in the heating furnace, the lower the Rayleigh scattering loss. By raising the heating temperature above 800 ° C., the transmission loss at a wavelength of 1.55 μm is reduced to 0. . 185 dB or less can be sufficiently reduced.

  For these reasons, the incoming temperature of the optical fiber in the heating furnace is made higher than 1500 ° C., the residence time of the optical fiber in the heating furnace is made longer than 0.1 second, and the heating temperature of the optical fiber in the heating furnace is made higher than 800 ° C. Thus, the Rayleigh scattering loss can be reduced, and even in a general single mode optical fiber, the transmission loss at a wavelength of 1.55 μm can be made smaller than 0.185 dB / km, and at a wavelength of 1.31 μm. Can be made smaller than 0.340 dB / km.

  Although such an annealing drawing technique can reduce the loss by reducing the Rayleigh scattering loss in the optical fiber, the loss at the wavelength of 1.38 μm is related to the radial distribution of the OH group concentration in the optical fiber and the propagation optical power. It is necessary to consider the OH absorption loss that depends on the radial distribution. The increase in transmission loss due to OH absorption loss at a wavelength of 1.38 μm is preferably 0.06 dB / km or less. However, in the annealing drawing, the OH group of the jacket layer diffuses to the vicinity of the core during the heating process in the heating furnace, and the OH absorption at a wavelength of 1.38 μm depends on the residence time of the optical fiber in the heating furnace. In some cases, the increase in transmission loss due to loss becomes large.

  FIG. 4 is a graph showing the relationship between the residence time of the optical fiber in the heating furnace and the transmission loss increase amount Δα1.38 derived from the OH absorption loss at a wavelength of 1.38 μm. Here, assuming that the radius of the core is a, the radius of the core rod is b, and Ra = b / a is defined, and when this Ra is 4.0 and 5.0, respectively, the OH absorption loss at a wavelength of 1.38 μm It shows the dependence of the increase in transmission loss Δα1.38 on the heating furnace residence time. In any case, the optical fiber preform has the same refractive index structure as that of a general general-purpose single mode optical fiber, the average OH concentration is less than 1 wtppb, and the thickness of the optical fiber after drawing from the surface. A jacket layer having an average OH concentration of 300 wtppm was provided around the core rod having an OH concentration of 1000 wtppm in a region where the thickness of 0.03 μm. The high OH concentration on the surface of the core rod is due to flame polishing with an oxyhydrogen flame or the like to clean the surface of the core rod when applying the jacket. The heating temperature of the optical fiber in the heating furnace was 1500 ° C.

  As shown in the figure, an optical fiber preform with a small Ra, that is, an optical fiber preform with a large ratio of the jacket portion to the core rod, has a large OH absorption loss of the optical fiber after drawing, There is a tendency for the OH absorption loss to increase as the furnace residence time increases. The reason why the OH absorption loss is increased is considered to be that OH groups existing in the core rod surface and the jacket diffuse to the vicinity of the core by heating. On the other hand, in the optical fiber preform with Ra of 5.0, the transmission loss increase amount Δα1.38 derived from the OH absorption loss is 0.02 dB / km or less even when the heating furnace stay time is long. This is considered to be because even if OH groups diffuse by heating, the diffused OH is sufficiently far from the core. From this, it is understood that when annealing drawing is performed, when Ra is small, the OH absorption loss changes according to the residence time of the optical fiber in the heating furnace. Need to be optimized.

  FIG. 5 is a graph showing the relationship between Ra of the optical fiber preform and transmission loss increase Δα1.38 derived from OH absorption loss at a wavelength of 1.38 μm. Again, the optical fiber preform shown in FIG. 4 was used. As shown in the figure, the shorter the residence time of the optical fiber in the heating furnace and the larger Ra, the smaller the transmission loss increase Δα1.38 derived from the OH absorption loss at the wavelength of 1.38 μm. When this optical fiber preform is used, the transmission derived from the OH absorption loss at a wavelength of 1.38 μm when the heating temperature of the optical fiber in the heating furnace is 1500 ° C. and the residence time of the optical fiber in the heating furnace is 1 second. In order to make the loss increase amount Δα1.38 0.06 dB / km or less, Ra needs to be 4.6 or more.

FIG. 6 is also a graph showing the relationship between Ra of the optical fiber preform and transmission loss increase Δα1.38 derived from OH absorption loss at a wavelength of 1.38 μm. Here, an optical fiber having a large core area and a large effective area was manufactured. The optical fiber had a core, a depressed surrounding the core, and a jacket surrounding the depressed. Based on the refractive index of the jacket, the relative refractive index difference of the core was 0.22%, and the relative refractive index difference of the depressed was -0.08%. The core radius was 6.5 μm. The effective area was 110 μm ² . When manufacturing an optical fiber having such a refractive index distribution, unlike the case of a general-purpose single mode optical fiber, by setting Ra to 2.7 or more, even when the heating furnace stay time is 1 second by annealing drawing. OH absorption loss can be kept low.

  Thus, both Rayleigh scattering loss and OH absorption loss can be reduced by optimizing Ra according to the refractive index structure of the optical fiber and performing annealing drawing. However, Ra for suppressing the OH absorption loss is different depending on the refractive index structure. Therefore, as a general index from Ra, the ratio of the power of the propagation light having a wavelength of 1.38 μm in the core rod equivalent region in the optical fiber (or the ratio of the power of the propagation light having a wavelength of 1.38 μm in the jacket equivalent region in the optical fiber). think of.

  FIG. 7 is a graph showing a relationship between the ratio of the power of propagating light having a wavelength of 1.38 μm in a jacket-corresponding region in an optical fiber and the increase in transmission loss Δα1.38 derived from OH absorption loss at a wavelength of 1.38 μm. . Here, the optical fiber preform shown in FIGS. 4 and 5 is denoted as structure 1, and the optical fiber preform shown in FIG. 6 is denoted as structure 2. As shown in the figure, regardless of the refractive index structure of the optical fiber, the ratio of the power of propagating light having a wavelength of 1.38 μm in the region corresponding to the core rod in the optical fiber is set to 99.99975% or more (that is, optical OH absorption loss can be reduced even if annealing is performed by setting the ratio of the power of propagating light having a wavelength of 1.38 μm in the region corresponding to the jacket of the fiber to 0.0025% or less.

  From the above, an optical fiber preform is produced by providing a jacket layer around the core rod to be a part of the core and clad of the optical fiber, and the optical fiber preform is annealed to produce an optical fiber. In the optical fiber preform, the OH concentration inside the core rod is 1 wtppb or less, the OH concentration 0.03 μm inside from the core rod surface is 100 to 1000 wtppm, and the OH group concentration of the jacket layer is 10 to 300 wt_ppm or more. To do. And after drawing this optical fiber preform in a drawing furnace to make an optical fiber, when annealing this optical fiber in a heating furnace, the heating temperature is set to 1500 ° C. or less, and the residence time is made shorter than 1 second. By setting the power ratio in the jacket portion to a predetermined value or less, an optical fiber in which both the Rayleigh scattering loss and the OH absorption loss are reduced can be manufactured at a low cost.

DESCRIPTION OF SYMBOLS 1 ... Optical fiber manufacturing apparatus, 2 ... Optical fiber preform, 3 ... Optical fiber, 4 ... Optical fiber strand, 11 ... Drawing furnace, 12 ... Heater, 13 ... Core tube, 14 ... Inert gas supply part, 15 ... inert gas supply path 21 ... heating furnace, 22 ... heater, 23 ... reactor core tube, 24 ... _{N 2} gas supply unit, 25 ... _{N 2} gas supply passage, 31 ... resin curing portion, 32 ... UV lamp, 41 ... Outer diameter measuring device, 42 ... drum, 43 ... drive motor, 44 ... control unit, 45 ... rotary drive shaft, 51 ... coating die, 52 ... UV resin liquid, 61 ... guide roller.

Claims (5)

A method of manufacturing an optical fiber having a core and a cladding, comprising:
A base material manufacturing step of manufacturing an optical fiber base material by providing a jacket layer to be the remainder of the cladding around the core rod to be a part of the core and the cladding,
A drawing process for producing an optical fiber by drawing the optical fiber preform with a drawing furnace;
A heating step of heating the optical fiber by a heating furnace;
A coating step of applying a coating layer around the optical fiber after being heated in the heating step;
With
In the base material manufacturing step, the average OH concentration is less than 1 wtppb, and the OH concentration in the region where the thickness of the optical fiber after drawing from the surface is 0.03 μm is 10 wtppm or more around the core rod. The optical fiber preform, wherein the jacket layer having a concentration of 10 wtppm or more is provided, and the ratio of the power of propagating light having a wavelength of 1.38 μm in the core rod equivalent region of the drawn optical fiber is 99.99975% or more Make
In the heating step, the temperature of the optical fiber when the optical fiber enters the heating furnace is 1400 ° C. or higher, the heating temperature of the optical fiber in the heating furnace is 2000 ° C. or lower, and the light in the heating furnace is The fiber stay time is 0.005 to 1 second.
An optical fiber manufacturing method.   In the base material manufacturing step, an average OH concentration is 10 to 300 wtppm or more around the core rod having an OH concentration of 100 to 1000 wtppm or more in a region where the thickness of the optical fiber after drawing from the surface is 0.03 μm. The optical fiber manufacturing method according to claim 1, wherein the optical fiber preform is manufactured by applying the jacket layer.   The optical fiber manufacturing method according to claim 2, wherein, in the heating step, a heating temperature of the optical fiber in the heating furnace is set to 800 to 1600 ° C or lower.   The optical fiber manufacturing method according to claim 2, wherein, in the heating step, the heating time of the optical fiber in the heating furnace is set to 0.10 to 1.00 seconds.   In the drawing step, an average glass cooling rate from the time when the glass outer diameter of the optical fiber preform starts to decrease to when the optical fiber exits from the drawing furnace is set to 500 ° C./m or more. An optical fiber manufacturing method according to claim 2.

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